The present invention relates to a method and apparatus for controlling the average heating power induced by an electromagnetic field in a moving or stationary, flat conducting product while maintaining the product in position (e.g. while levitating or guiding the product) without contact, by electromagnetic forces also induced by the heat inducing field, particularly in an electromagnetic heating installation comprising a pair of essentially identical inductors having poles facing each other in pairs on either side of the product.
In various finishing operations and especially in certain heat treatment processes, it may be necessary to regulate the power induced in a product in order to follow a predetermined temperature curve as a function of time, while maintaining the position of the product without contact, for example in levitation.
It is obvious that to reduce the power induced in a product exposed to a transverse flux, it may be sufficient to reduce the intensity of the current flowing through the inductors creating the flux, but when the product is in levitation with conventional devices, the reduction involves a reduction in the height of levitation. Furthermore, if it is desired to position the product without contact, the reduction in power is necessarily related to the need to maintain the height of levitation at a minimum value.
The object of the present invention is to control over the broadest range possible the heating power electromagnetically induced in a flat, conducting product supported without contact (e.g. in levitation or guidance), by the correlatively induced electromagnetic forces, while insuring an essentially constant position of the product and by using a simple and economical apparatus.
It is known that in an electromagnetic heating installation with transverse flux, comprising a pair of identical inductors placed on either side of a product supported electromagnetically without contact with the poles of one of the inductors of the pair facing the poles of the other inductor and supplied with, for example, a single phase alternating current, the power induced in the product is at a maximum when the poles facing each other in pairs are at all times at opposite magnetic polarities and when the magnetic flux intensities passing through the inductors are at a maximum. But it is also known that the power induced is nearly annulled (for example at a ratio of 1000 to 1) when the poles facing each other in pairs are at all times of the same polarity and when the magnetic flux intensities passing through the inductors are at a maximum. This phenomenon and an apparatus operating in this manner are more fully described in a U.S. patent application by Jean-Claude Bronner for "Method And Apparatus For Minimizing The Power Induced In A Flat Conducting Product Maintained In Position Electromagnetically Without Contact, Ser. No. 464,952," filed concurrently herewith, assigned to the assignee of the present invention and hereby incorporated herein by reference.
The process according to the invention, for controlling the heating power induced by an electromagnetic field in a flat, conducting product supported without contact by the electromagnetic forces correlatively induced by the heat inducing field, involves an installation for heating by electromagnetic induction wherein an essentially identical pair of inductors are placed on either side of the product with the poles of one inductor of the pair facing the poles of the other inductor. A sequence of inversions of the instantaneous polarity of the poles of one of the inductors with respect to the instantaneous polarity of the corresponding poles of the other inductor is effected, with the ratio of the durations during which the magnetic polarities of the poles of the one inductor are on the one hand opposite and on the other identical to the magnetic polarities of the corresponding poles of the other inductor, determining the average value of the heating power induced, the position of the product remaining practically constant.
In actual fact, by means of calculations and by experiments, the present applicant has discovered that quite unexpectedly, the electromagnetic forces applied to the product and which maintain it without contact, remain essentially the same whether the instantaneous magnetic polarities of the pairs of inductor poles facing each other are all identical or all opposite, all other conditions being equal, and that during an inversion of the relative polarity of the poles of one of the inductors with respect to the polarity of the corresponding poles of the other inductor, the position of the product remains practically unchanged.
One of the advantages of the process according to the invention originates in the fact that the distances separating the polar surfaces of each pair of poles facing each other from either side of the product, may be reduced to a minimum corresponding to the space required by the overall dimensions in thickness of the product taking into account its deformations and possible existing thermal insulation, with the clearance needed to insure the absence of contact being nearly zero. The yield and the power factor of the means effecting the process are thereby improved.
This simple and economical process is applicable particularly to the reduction of the power induced at the end of heating. As dynamic temperature deviations are proportional to the power induced in relation to the product volume and result both from conduction phenomena and the not perfectly uniform distribution of the current densities induced, this reduction in power makes it possible to obtain smaller dynamic temperature deviations and thus, for example, to attain a relatively uniform average temperature in the product without locally exceeding the temperature limit to be observed.
The invention is applicable especially to installations for heating in levitation, for example heating with electromagnetic guidance, where it is necessary to maintain essentially constant forces tending to return the product toward a position of equilibrium.
The invention further concerns an apparatus for controlling the average power induced electromagnetically in a flat, conducting product, maintained without contact in an essentially constant position by correlatively induced electromagnetic forces, the apparatus effecting the process according to the invention and comprising a means to invert the magnetic polarity of selected poles of inductors on either side of the product, the apparatus further including, depending on the particular case:
a control system;
means to sense the temperature of the product;
means to sense the power induced;
means defining a power cycle to be effected;
means defining a temperature cycle to be effected;
the information, results and data obtained by the foregoing means, together with the properties of the product which are determinative of its induction heating characteristics being taken into account by the control system in order to control the heating of the product.
The inversions of the instantaneous magnetic polarity of the poles of one of the inductors with respect to the instantaneous magnetic polarity of the corresponding poles of the inductor are accomplished in any suitable, conventional manner by a polarity inverter performing, for example, an inversion of the supply line of one of the inductors at the alternating source of current or potential (the potential/current ratio is different according to whether the polarities are identical or opposed).
A device according to the invention includes, in an alternate embodiment, a conventional electronic curve generating and following circuitry such as a cyclic ratio generator, the frequency of which is a function of the precision of control desired, in order to make it possible to observe in a rigorous fashion a predetermined curve as a function of time.
In accordance with a preferred embodiment, the polarity inverter is of the static type, comprising two rectifier bridges connected so as to be individually selectable to supply power to the coils of the inductor poles .